FIG. 1 schematically shows an optical modulator according to the Mach-Zehnder interferometer principle, commonly referred to as an MZI modulator. The modulator includes an optical waveguide that receives power Pin, which is divided into two branches 12a and 12b at a point S. The two branches come together again at a point J. Each branch carries half of the original optical power.
Each branch comprises a static electro-optical phase shifter SPS (SPSa and SPSb) and a dynamic electro-optical phase shifter DPS (DPSa and DPSb). The static phase shifters SPS are used to define an initial phase difference φ0 between the two optical waveguide branches. They are controlled by respective bias signals IBa and IBb. The dynamic phase shifters DPS are used to perform a differential modulation around the initial conditions defined by the SPS phase shifters. They are controlled by respective modulation signals M and M/ varying in phase opposition.
The waves arriving on both branches of the modulator are added at point J. The resulting wave has a power of Pin·cos2(Δφ/2), neglecting the optical losses, where Δφ is the instantaneous phase difference between the waves of the two branches.
FIG. 2 is a perspective view of the waveguide branches 12a and 12b incorporating phase shifters SPS and DPS, shown in gray. As shown, the waveguides are formed in transparent islands, made of intrinsic semiconductor material, having an inverted “T” section, the central portion of which transmits the optical beam. The phase shifters are configured to replace waveguide segments and have the same inverted “T” cross section. The edges of the phase shifters bear electrical contacts used to control the phase shifters—they usually extend above the plane of the waveguide, as shown, to reach the metal levels.
FIG. 3A is a schematic sectional view of a DPS phase shifter referred to as a High-Speed Phase Modulator (HSPM). The cross section plane is perpendicular to the axis of the optical waveguide. A dotted circle, at the thicker central region, represents the region crossed by the optical beam, which is hereinafter referred to as the “optical action zone”.
The phase shifter comprises a semiconductor structure, typically silicon, forming a P-N junction 14 in a plane parallel to the axis of the waveguide, and offset relative thereto. The junction 14 is shown, for example, at the right side face of the waveguide.
A P-doped zone extends to the left of junction 14, which has a cross section conforming to the cross section of the waveguide, namely elevated in the center and lower at the edge. Zone P ends at its left by a P+ doped raised area, bearing an anode contact A.
An N-doped zone extends to the right of the junction 14, conforming to the cross section of the waveguide. The zone N ends to the right by an N+ doped raised area, bearing a cathode contact C. The structure of the phase shifter may be formed on an insulating substrate, for example a buried oxide layer BOX.
For controlling the phase shifter of FIG. 3A, a voltage is applied between the anode and cathode contacts A, C, which reverse biases the junction 14 (the ‘+’ on the cathode and the ‘−’ on the anode). This configuration causes a displacement of electrons e from the N region to the cathode and of holes h from the P region to the anode, and the creation of a depletion region D in the vicinity of the junction 14. The carrier concentration is thus modified in accordance with the magnitude of the bias voltage in the area crossed by the optical beam, which results in a corresponding modification of the refractive index of this area. More specifically, this type of phase shifter has a negative coefficient in that an increase in bias voltage causes a decrease in the phase shift.
FIG. 3B is a schematic sectional view of a P-I-N junction SPS phase shifter. The P and N-doped central regions of the structure of FIG. 3A have been replaced by a single intrinsic semiconductor zone I, which, in practice, is a zone having a minimal P doping level. For controlling this phase shifter, a current is applied between the anode and cathode contacts A and C, which forward biases the junction (the ‘−’ on the cathode and the ‘+’ on the anode). A current establishes between the anode and the cathode causing the injection of carriers in the intrinsic zone I (holes h from the P+ region to zone I and electrons e from the N+ region to zone I). Thus, changes in the current induce changes in the carrier concentration, which in turn modify the refractive index of the optical action zone. More specifically, this type of phase shifter has a positive coefficient in that an increase of the bias current results in an increase of the phase shift.
PIN phase shifters typically have a slow response compared to HSPM shifters, but they offer a wider range of adjustment, which is why they may be used to set the quiescent conditions of the modulator.